Field of the Invention
The present invention relates to a semiconductor device that has circuits formed from thin film transistors (hereinafter referred to as TFT) on a substrate having an insulating surface, and to a method of manufacturing the same. More specifically, the present invention is suitably applied to an electro-optical device represented by a liquid crystal display device in which a pixel portion (or, a pixel matrix circuit) and a driver circuit provided in the periphery thereof are formed on the same single substrate, and to an electronic equipment equipped with such electro-optical device. Incidentally, the `semiconductor device` in this specification refers to devices in general which utilizes semiconductor characteristics to function and, therefore, the electro-optical device and the electronic equipment equipped with the electro-optical device mentioned above are contained in the category.
The progress has been made in developing a semiconductor device that has circuits formed from TFTs on substrates having an insulating surface. Active matrix liquid crystal display devices are well known as a typical example of such semiconductor device. In particular, great effort is put into the development of the electro-optical device with TFTs whose active layers are made of crystalline silicon films (hereinafter referred to as crystalline silicon TFT) integrally formed on the same single substrate, for these TFTs are high in field effect mobility and hence afford to form various functional circuits.
For instance, an active matrix liquid crystal display device is provided with a pixel portion displaying an image, a driver circuit used to display an image, etc. The driver circuit is comprised of circuits formed by using CMOS circuits as the base, such as a shift register circuit, a level shifter circuit, a buffer circuit, and a sampling circuit. Those circuits are mixedly mounted on the same single substrate.
When taking a look at those circuits separately, one does not always share its operation condition with others, which causes no small difference in characteristics required for the TFTs. For example, the pixel portion is comprised of a pixel TFT formed from an N channel TFT and of a holding capacitor, and is driven by applying voltage to liquid crystal while using the pixel TFT as a switching element. Since liquid crystal is driven with alternating current, a system called frame inversion driving is often used. In this system, characteristic required for the pixel TFT to keep power consumption low is to reduce sufficiently the OFF current value (drain current flowing at the time of OFF operation of the TFT). On the other hand, in the driver circuit, the withstand voltage has to be enhanced lest its buffer circuit to which a high driving voltage is applied is broken upon application of the high voltage. Also, securing enough ON current (drain current flowing at the time of OFF operation of the TFT) is required to enhance current drive performance.
However, there is a problem in that the OFF current of crystalline silicon TFTs tend to take a large value. In addition, degradation phenomena such as lowering of ON current value are observed in crystalline silicon TFTs, similar to MOS transistors used in ICs and the like. The main cause of the phenomena could be hot carrier injection: it is surmised that hot carriers generated by the high electric field in the vicinity of the drain bring about the degradation phenomena.
A TFT structure known as useful in reducing OFF current is the lightly doped drain (LDD) structure. According to this structure, a region doped with an impurity element in a low concentration is formed between a channel formation region and a source region, or a drain region, that is doped with a high concentration of impurity element. This lightly doped region is called LDD region.
Also known as measures for preventing the degradation brought by hot carriers is a `GOLD` structure (Gate-drain Overlapped LDD) in which the LDD region is arranged so as to overlap with a gate electrode through a gate insulating film. These structures release the high electric field in the vicinity of the drain to prevent the hot carrier injection, and hence is effective in preventing the degradation phenomena. For instance, an article written by Mutuko Hatano, Hajime Akimoto and Takeshi Sakai in IEDM97 TECHNICAL DIGEST on pages 523 to 526 in 1997 discloses a GOLD structure formed from side walls of silicon, which verifies that very excellent reliability can be obtained with the GOLD structure compared to TFTs having other structure.
Required characteristics, however, is not always the same for the pixel TFT of the pixel portion and for the TFTs of the driver circuit, such as the shift register or the buffer circuit. To give an example, a large reverse bias (negative voltage in the N channel TFT) is applied to the gate in the pixel TFT while the TFTs of the driver circuit do not basically operate under the reverse bias state. Also, the pixel TFT operates at a speed 1/100 times the operation speed of the TFTs in the driver circuit.
In addition, GOLD structures have a problem in that, though high in the effect to prevent the degradation of ON current value, OFF current value is larger than in usual LDD structures. Thus the GOLD structures are not preferable in application to the pixel TFT. On the other hand, usual LDD structures are high in the effect to suppress OFF current value but is low in the effect to release the electric field in the vicinity of the drain and prevent the degradation due to hot carriers. It is thus not always preferable to form all TFTs to have the same structure in a semiconductor device that has a plurality of integrated circuits different from one another in the operation condition, as in active matrix liquid crystal display devices. The problem as such comes to the front especially as the characteristics of crystalline silicon TFTs are enhanced and more is demanded for the performance of active matrix liquid crystal display devices.